github.com/krum110487/go-htaccess@v0.0.0-20240316004156-60641c8e7598/tests/data/apache_2_2_34/manual/dso.html.en (about) 1 <?xml version="1.0" encoding="ISO-8859-1"?> 2 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> 3 <html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en"><head> 4 <meta content="text/html; charset=ISO-8859-1" http-equiv="Content-Type" /> 5 <!-- 6 XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX 7 This file is generated from xml source: DO NOT EDIT 8 XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX 9 --> 10 <title>Dynamic Shared Object (DSO) Support - Apache HTTP Server Version 2.2</title> 11 <link href="./style/css/manual.css" rel="stylesheet" media="all" type="text/css" title="Main stylesheet" /> 12 <link href="./style/css/manual-loose-100pc.css" rel="alternate stylesheet" media="all" type="text/css" title="No Sidebar - Default font size" /> 13 <link href="./style/css/manual-print.css" rel="stylesheet" media="print" type="text/css" /><link rel="stylesheet" type="text/css" href="./style/css/prettify.css" /> 14 <script src="./style/scripts/prettify.min.js" type="text/javascript"> 15 </script> 16 17 <link href="./images/favicon.ico" rel="shortcut icon" /><link href="http://httpd.apache.org/docs/current/dso.html" rel="canonical" /></head> 18 <body id="manual-page"><div id="page-header"> 19 <p class="menu"><a href="./mod/">Modules</a> | <a href="./mod/directives.html">Directives</a> | <a href="http://wiki.apache.org/httpd/FAQ">FAQ</a> | <a href="./glossary.html">Glossary</a> | <a href="./sitemap.html">Sitemap</a></p> 20 <p class="apache">Apache HTTP Server Version 2.2</p> 21 <img alt="" src="./images/feather.gif" /></div> 22 <div class="up"><a href="./"><img title="<-" alt="<-" src="./images/left.gif" /></a></div> 23 <div id="path"> 24 <a href="http://www.apache.org/">Apache</a> > <a href="http://httpd.apache.org/">HTTP Server</a> > <a href="http://httpd.apache.org/docs/">Documentation</a> > <a href="./">Version 2.2</a></div><div id="page-content"><div class="retired"><h4>Please note</h4> 25 <p> This document refers to a legacy release (<strong>2.2</strong>) of Apache httpd. The active release (<strong>2.4</strong>) is documented <a href="http://httpd.apache.org/docs/current">here</a>. If you have not already upgraded, please follow <a href="http://httpd.apache.org/docs/current/upgrading.html">this link</a> for more information.</p> 26 <p>You may follow <a href="http://httpd.apache.org/docs/current/dso.html">this link</a> to go to the current version of this document.</p></div><div id="preamble"><h1>Dynamic Shared Object (DSO) Support</h1> 27 <div class="toplang"> 28 <p><span>Available Languages: </span><a href="./en/dso.html" title="English"> en </a> | 29 <a href="./ja/dso.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> | 30 <a href="./ko/dso.html" hreflang="ko" rel="alternate" title="Korean"> ko </a> | 31 <a href="./tr/dso.html" hreflang="tr" rel="alternate" title="Türkçe"> tr </a></p> 32 </div> 33 34 <p>The Apache HTTP Server is a modular program where the 35 administrator can choose the functionality to include in the 36 server by selecting a set of modules. The modules can be 37 statically compiled into the <code class="program"><a href="./programs/httpd.html">httpd</a></code> binary when the 38 server is built. Alternatively, modules can be compiled as 39 Dynamic Shared Objects (DSOs) that exist separately from the 40 main <code class="program"><a href="./programs/httpd.html">httpd</a></code> binary file. DSO modules may be 41 compiled at the time the server is built, or they may be 42 compiled and added at a later time using the Apache Extension 43 Tool (<code class="program"><a href="./programs/apxs.html">apxs</a></code>).</p> 44 45 <p>This document describes how to use DSO modules as well as 46 the theory behind their use.</p> 47 </div> 48 <div id="quickview"><ul id="toc"><li><img alt="" src="./images/down.gif" /> <a href="#implementation">Implementation</a></li> 49 <li><img alt="" src="./images/down.gif" /> <a href="#usage">Usage Summary</a></li> 50 <li><img alt="" src="./images/down.gif" /> <a href="#background">Background</a></li> 51 <li><img alt="" src="./images/down.gif" /> <a href="#advantages">Advantages and Disadvantages</a></li> 52 </ul><ul class="seealso"><li><a href="#comments_section">Comments</a></li></ul></div> 53 <div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div> 54 <div class="section"> 55 <h2><a name="implementation" id="implementation">Implementation</a></h2> 56 57 <table class="related"><tr><th>Related Modules</th><th>Related Directives</th></tr><tr><td><ul><li><code class="module"><a href="./mod/mod_so.html">mod_so</a></code></li></ul></td><td><ul><li><code class="directive"><a href="./mod/mod_so.html#loadmodule">LoadModule</a></code></li></ul></td></tr></table> 58 59 <p>The DSO support for loading individual Apache modules is based 60 on a module named <code class="module"><a href="./mod/mod_so.html">mod_so</a></code> which must be statically 61 compiled into the Apache core. It is the only module besides 62 <code class="module"><a href="./mod/core.html">core</a></code> which cannot be put into a DSO 63 itself. Practically all other distributed Apache modules can then 64 be placed into a DSO by individually enabling the DSO build for 65 them via <code class="program"><a href="./programs/configure.html">configure</a></code>'s 66 <code>--enable-<em>module</em>=shared</code> option as discussed 67 in the <a href="install.html">install documentation</a>. After a 68 module is compiled into a DSO named <code>mod_foo.so</code> you 69 can use <code class="module"><a href="./mod/mod_so.html">mod_so</a></code>'s <code class="directive"><a href="./mod/mod_so.html#loadmodule">LoadModule</a></code> command in your 70 <code>httpd.conf</code> file to load this module at server startup 71 or restart.</p> 72 73 <p>To simplify this creation of DSO files for Apache modules 74 (especially for third-party modules) a new support program 75 named <code class="program"><a href="./programs/apxs.html">apxs</a></code> (<dfn>APache 76 eXtenSion</dfn>) is available. It can be used to build DSO based 77 modules <em>outside of</em> the Apache source tree. The idea is 78 simple: When installing Apache the <code class="program"><a href="./programs/configure.html">configure</a></code>'s 79 <code>make install</code> procedure installs the Apache C 80 header files and puts the platform-dependent compiler and 81 linker flags for building DSO files into the <code class="program"><a href="./programs/apxs.html">apxs</a></code> 82 program. This way the user can use <code class="program"><a href="./programs/apxs.html">apxs</a></code> to compile 83 his Apache module sources without the Apache distribution 84 source tree and without having to fiddle with the 85 platform-dependent compiler and linker flags for DSO 86 support.</p> 87 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div> 88 <div class="section"> 89 <h2><a name="usage" id="usage">Usage Summary</a></h2> 90 91 <p>To give you an overview of the DSO features of Apache 2.x, 92 here is a short and concise summary:</p> 93 94 <ol> 95 <li> 96 Build and install a <em>distributed</em> Apache module, say 97 <code>mod_foo.c</code>, into its own DSO 98 <code>mod_foo.so</code>: 99 100 <div class="example"><p><code> 101 $ ./configure --prefix=/path/to/install --enable-foo=shared<br /> 102 $ make install 103 </code></p></div> 104 </li> 105 106 <li> 107 Build and install a <em>third-party</em> Apache module, say 108 <code>mod_foo.c</code>, into its own DSO 109 <code>mod_foo.so</code>: 110 111 <div class="example"><p><code> 112 $ ./configure --add-module=<var>module_type</var>:/path/to/3rdparty/mod_foo.c \<br /> 113 <span class="indent"> 114 --enable-foo=shared<br /> 115 </span> 116 $ make install 117 </code></p></div> 118 </li> 119 120 <li> 121 Configure Apache for <em>later installation</em> of shared 122 modules: 123 124 <div class="example"><p><code> 125 $ ./configure --enable-so<br /> 126 $ make install 127 </code></p></div> 128 </li> 129 130 <li> 131 Build and install a <em>third-party</em> Apache module, say 132 <code>mod_foo.c</code>, into its own DSO 133 <code>mod_foo.so</code> <em>outside of</em> the Apache 134 source tree using <code class="program"><a href="./programs/apxs.html">apxs</a></code>: 135 136 <div class="example"><p><code> 137 $ cd /path/to/3rdparty<br /> 138 $ apxs -c mod_foo.c<br /> 139 $ apxs -i -a -n foo mod_foo.la 140 </code></p></div> 141 </li> 142 </ol> 143 144 <p>In all cases, once the shared module is compiled, you must 145 use a <code class="directive"><a href="./mod/mod_so.html#loadmodule">LoadModule</a></code> 146 directive in <code>httpd.conf</code> to tell Apache to activate 147 the module.</p> 148 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div> 149 <div class="section"> 150 <h2><a name="background" id="background">Background</a></h2> 151 152 <p>On modern Unix derivatives there exists a nifty mechanism 153 usually called dynamic linking/loading of <em>Dynamic Shared 154 Objects</em> (DSO) which provides a way to build a piece of 155 program code in a special format for loading it at run-time 156 into the address space of an executable program.</p> 157 158 <p>This loading can usually be done in two ways: Automatically 159 by a system program called <code>ld.so</code> when an 160 executable program is started or manually from within the 161 executing program via a programmatic system interface to the 162 Unix loader through the system calls 163 <code>dlopen()/dlsym()</code>.</p> 164 165 <p>In the first way the DSO's are usually called <em>shared 166 libraries</em> or <em>DSO libraries</em> and named 167 <code>libfoo.so</code> or <code>libfoo.so.1.2</code>. They 168 reside in a system directory (usually <code>/usr/lib</code>) 169 and the link to the executable program is established at 170 build-time by specifying <code>-lfoo</code> to the linker 171 command. This hard-codes library references into the executable 172 program file so that at start-time the Unix loader is able to 173 locate <code>libfoo.so</code> in <code>/usr/lib</code>, in 174 paths hard-coded via linker-options like <code>-R</code> or in 175 paths configured via the environment variable 176 <code>LD_LIBRARY_PATH</code>. It then resolves any (yet 177 unresolved) symbols in the executable program which are 178 available in the DSO.</p> 179 180 <p>Symbols in the executable program are usually not referenced 181 by the DSO (because it's a reusable library of general code) 182 and hence no further resolving has to be done. The executable 183 program has no need to do anything on its own to use the 184 symbols from the DSO because the complete resolving is done by 185 the Unix loader. (In fact, the code to invoke 186 <code>ld.so</code> is part of the run-time startup code which 187 is linked into every executable program which has been bound 188 non-static). The advantage of dynamic loading of common library 189 code is obvious: the library code needs to be stored only once, 190 in a system library like <code>libc.so</code>, saving disk 191 space for every program.</p> 192 193 <p>In the second way the DSO's are usually called <em>shared 194 objects</em> or <em>DSO files</em> and can be named with an 195 arbitrary extension (although the canonical name is 196 <code>foo.so</code>). These files usually stay inside a 197 program-specific directory and there is no automatically 198 established link to the executable program where they are used. 199 Instead the executable program manually loads the DSO at 200 run-time into its address space via <code>dlopen()</code>. At 201 this time no resolving of symbols from the DSO for the 202 executable program is done. But instead the Unix loader 203 automatically resolves any (yet unresolved) symbols in the DSO 204 from the set of symbols exported by the executable program and 205 its already loaded DSO libraries (especially all symbols from 206 the ubiquitous <code>libc.so</code>). This way the DSO gets 207 knowledge of the executable program's symbol set as if it had 208 been statically linked with it in the first place.</p> 209 210 <p>Finally, to take advantage of the DSO's API the executable 211 program has to resolve particular symbols from the DSO via 212 <code>dlsym()</code> for later use inside dispatch tables 213 <em>etc.</em> In other words: The executable program has to 214 manually resolve every symbol it needs to be able to use it. 215 The advantage of such a mechanism is that optional program 216 parts need not be loaded (and thus do not spend memory) until 217 they are needed by the program in question. When required, 218 these program parts can be loaded dynamically to extend the 219 base program's functionality.</p> 220 221 <p>Although this DSO mechanism sounds straightforward there is 222 at least one difficult step here: The resolving of symbols from 223 the executable program for the DSO when using a DSO to extend a 224 program (the second way). Why? Because "reverse resolving" DSO 225 symbols from the executable program's symbol set is against the 226 library design (where the library has no knowledge about the 227 programs it is used by) and is neither available under all 228 platforms nor standardized. In practice the executable 229 program's global symbols are often not re-exported and thus not 230 available for use in a DSO. Finding a way to force the linker 231 to export all global symbols is the main problem one has to 232 solve when using DSO for extending a program at run-time.</p> 233 234 <p>The shared library approach is the typical one, because it 235 is what the DSO mechanism was designed for, hence it is used 236 for nearly all types of libraries the operating system 237 provides. On the other hand using shared objects for extending 238 a program is not used by a lot of programs.</p> 239 240 <p>As of 1998 there are only a few software packages available 241 which use the DSO mechanism to actually extend their 242 functionality at run-time: Perl 5 (via its XS mechanism and the 243 DynaLoader module), Netscape Server, <em>etc.</em> Starting 244 with version 1.3, Apache joined the crew, because Apache 245 already uses a module concept to extend its functionality and 246 internally uses a dispatch-list-based approach to link external 247 modules into the Apache core functionality. So, Apache is 248 really predestined for using DSO to load its modules at 249 run-time.</p> 250 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div> 251 <div class="section"> 252 <h2><a name="advantages" id="advantages">Advantages and Disadvantages</a></h2> 253 254 <p>The above DSO based features have the following 255 advantages:</p> 256 257 <ul> 258 <li>The server package is more flexible at run-time because 259 the actual server process can be assembled at run-time via 260 <code class="directive"><a href="./mod/mod_so.html#loadmodule">LoadModule</a></code> 261 <code>httpd.conf</code> configuration commands instead of 262 <code class="program"><a href="./programs/configure.html">configure</a></code> options at build-time. For instance 263 this way one is able to run different server instances 264 (standard & SSL version, minimalistic & powered up 265 version [mod_perl, PHP3], <em>etc.</em>) with only one Apache 266 installation.</li> 267 268 <li>The server package can be easily extended with 269 third-party modules even after installation. This is at least 270 a great benefit for vendor package maintainers who can create 271 a Apache core package and additional packages containing 272 extensions like PHP3, mod_perl, mod_fastcgi, 273 <em>etc.</em></li> 274 275 <li>Easier Apache module prototyping because with the 276 DSO/<code class="program"><a href="./programs/apxs.html">apxs</a></code> pair you can both work outside the 277 Apache source tree and only need an <code>apxs -i</code> 278 command followed by an <code>apachectl restart</code> to 279 bring a new version of your currently developed module into 280 the running Apache server.</li> 281 </ul> 282 283 <p>DSO has the following disadvantages:</p> 284 285 <ul> 286 <li>The DSO mechanism cannot be used on every platform 287 because not all operating systems support dynamic loading of 288 code into the address space of a program.</li> 289 290 <li>The server is approximately 20% slower at startup time 291 because of the symbol resolving overhead the Unix loader now 292 has to do.</li> 293 294 <li>The server is approximately 5% slower at execution time 295 under some platforms because position independent code (PIC) 296 sometimes needs complicated assembler tricks for relative 297 addressing which are not necessarily as fast as absolute 298 addressing.</li> 299 300 <li>Because DSO modules cannot be linked against other 301 DSO-based libraries (<code>ld -lfoo</code>) on all platforms 302 (for instance a.out-based platforms usually don't provide 303 this functionality while ELF-based platforms do) you cannot 304 use the DSO mechanism for all types of modules. Or in other 305 words, modules compiled as DSO files are restricted to only 306 use symbols from the Apache core, from the C library 307 (<code>libc</code>) and all other dynamic or static libraries 308 used by the Apache core, or from static library archives 309 (<code>libfoo.a</code>) containing position independent code. 310 The only chances to use other code is to either make sure the 311 Apache core itself already contains a reference to it or 312 loading the code yourself via <code>dlopen()</code>.</li> 313 </ul> 314 315 </div></div> 316 <div class="bottomlang"> 317 <p><span>Available Languages: </span><a href="./en/dso.html" title="English"> en </a> | 318 <a href="./ja/dso.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> | 319 <a href="./ko/dso.html" hreflang="ko" rel="alternate" title="Korean"> ko </a> | 320 <a href="./tr/dso.html" hreflang="tr" rel="alternate" title="Türkçe"> tr </a></p> 321 </div><div class="top"><a href="#page-header"><img src="./images/up.gif" alt="top" /></a></div><div class="section"><h2><a id="comments_section" name="comments_section">Comments</a></h2><div class="warning"><strong>Notice:</strong><br />This is not a Q&A section. 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